Cognitive radio is known as a technology which recognizes the wireless environment of the surroundings and optimizes communication parameters according to the wireless environment. An example of cognitive radio system is where a secondary system (interference giving system) shares a frequency band assigned to a primary system (interference receiving system).
For example, standardization of a wireless regional area network (Wireless Regional Area Network: WRAN) system as a secondary system sharing a frequency band (TV channel) assigned to a TV broadcasting system as a primary system is discussed in IEEE (Institute of Electrical and Electronic Engineers) 802.22 Working Group.
A WRAN system uses spectrum sensing technology to judge a primary system uses a frequency band is used (refer to non-patent literature 1). Here, spectrum sensing indicates reception processing in which a secondary system receives a radio wave of a shared frequency band and which recognizes whether a signal of a primary system exists. A WRAN system as a secondary system needs to check whether a primary system is not communicating in a shared frequency band. In order to check it periodically, a transmission suspension period called a Quiet Period (abbreviated as QP hereafter) is used in the WRAN system, and spectrum sensing is performed within the QP.
FIG. 19 is a figure which illustrates a frame structure of a QP in a WRAN system, in which a QP is inserted in a transmission signal periodically. Also, each QP in a plurality of transmitting stations of the WRAN system is synchronized in the time domain. Spectrum sensing with such a QP makes it possible to sense a signal of a primary system (primary signal) even if it is a weak signal because a WRAN signal (secondary signal) is not transmitted. Accordingly, accuracy of spectrum sensing can be improved.
As mentioned above, in cognitive radio, a secondary system checks whether a primary system in the vicinity is using the frequency band. Therefore, it is necessary to grasp an amount of interference in the secondary system by the use of spectrum sensing, for example, the spectrum sensing using QP mentioned above. On the other hand, it is also important to grasp how much interference is caused (amount of interference) to the primary system by the transmission of the secondary system. That is, when a secondary system shares a frequency band with a primary system, it is necessary for the secondary system to avoid excessive interference to the primary system which exists in the vicinity. It is also necessary for the secondary system not to have an influence on existing services which the primary system provides. An amount of interference to the primary system, for example, can be used for determination of transmission power of the secondary system that is necessary to suppress influence of interference in the primary system within a fixed value.
FIG. 20 is a figure illustrating interference between systems in a general wireless communication system including a primary system and a secondary system. As can be understood from FIG. 20, transmitting station 20 (secondary transmitting station) of a secondary system which performs spectrum sensing suffers interference (suffered interference) by the signal transmitted from transmitting station 10 (primary transmitting station) of a primary system. Also, the signal transmitted from secondary transmitting station 20 causes interference to receiving station 11 (primary receiving station) of the primary system (given interference).
As an example of such technology that grasps an amount of interference to a primary receiving station, technology disclosed in non-patent literature 2 can be mentioned. In non-patent literature 2, the technology which estimates given interference to a primary system is disclosed by specifying a service area of a TV which is a primary system using a TV database and by using propagation loss at a service area border which is estimated based on a propagation model.
Also, as an example of technology in general which grasps an amount of interference, technology disclosed in patent document 1 can be mentioned. In patent document 1, technology which measures interference from a peripheral cell (for example, an adjacent cell) in a cellular system is disclosed. A cell which measures interference from an adjacent cell suspends a signal transmission by installing a space frame (which is a transmission suspension period and corresponds to a QP of a WRAN system) in each of an uplink signal and a downlink signal. The cell measures an amount of suffered interference by the transmission of the adjacent cell in a state that it is not influenced by the transmission of its own cell with the suspension of the signal transmission. Also, an amount of interference from its own cell to the adjacent cell can be measured by using a space frame similarly in the adjacent cell and by the adjacent cell measuring an amount of suffered interference.    Patent document 1: Japanese Patent Application Laid-Open No. 2000-138629    Non-patent literature 1: C. Cordeiro, M. Ghosh, D. Cavalcanti, K. Challapali, “Spectrum sensing for Dynamic Spectrum Access of TV Bands”, Proc. IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Network (DySPAN), October 2008.    Non-patent literature 2: D. Gurney, G. Buchwald, L. Ecklund, S. Kuffner, and J. Grosspietsch, “Geo-location database techniques for incumbent protection in the TV white space”, Proc. International Conference on Cognitive Radio Oriented Wireless Access Networks and Communications (CrownCom), July 2007.